1. Field of the Invention
The invention is an amplifying compensation circuit for a semiconductor pressure sensor. More particularly, the invention relates to an amplifying compensation circuit which realizes a high precision semiconductor pressure sensor with a small number of circuit elements.
2. Description of the Prior Art
A semiconductor pressure sensor generates a voltage or a set of voltages which are dependent on the pressure applied to the sensor. Circuitry is provided to amplify the generated voltage and to adjust the generated voltage.
The voltage generated by a semiconductor pressure sensor may vary with the temperature. Therefore, circuitry associated with the semiconductor pressure sensor must compensate for temperature induced voltage changes to achieve accurate pressure readings.
Different semiconductor pressure sensors will have slightly different voltage characteristics depending on the manufacturing attributes. Therefore, the sensitivity of the pressure sensor is adjusted.
The zero point is the semiconductor pressure sensor output voltage when no external pressure or only a background pressure is applied to the pressure sensor. The zero point may vary with the temperature. Therefore, circuitry should be provided to compensate for the temperature induced changes in the zero point.
The setting of the zero point should be adjustable to allow the pressure sensor to operate over a wide range of conditions.
FIG. 4 shows one example of a circuit for performing the two compensations and two adjustments discussed above. The output voltage of a bridge 100 made up of strain gauges 101-104 is dependent on the pressure applied to the strain gauges 101-104. The output voltage is amplified by a front stage differential amplifier comprising operational amplifiers 201 and 202 and resistors 1-3 and is then amplified by a rear stage differential amplifier comprising an operational amplifier 203 and resistors 4-7 and 52. The output voltage thus amplified is provided as an output V.sub.out.
Resistor 52 compensates for temperature induced voltage changes. Resistor 52 is a diffusion resistor which has a positive temperature dependency. Resistor 52 gives a positive temperature dependency to the degree of amplification of the rear stage differential amplifier and, therefore, compensates the negative temperature dependency of the strain gauge bridge. In the compensation circuit, the resistor 7 serves as an adjustment resistor.
Sensitivity adjustment is carried out by adjusting the resistance of the resistor 1 until the amplification of the circuit is a predetermined value.
The output voltage Vd of an operational amplifier 204 is added to the sensor output voltage V.sub.out of the rear stage differential amplifier including the operational amplifier 203. The potential and temperature characteristics of the output voltage Vd of the amplifier 204 are selected to perform zero point adjustment and zero point temperature dependent change compensation.
In FIG. 4, resistors 50 and 51 have temperature dependent resistance. The resistances of resistors 10 and 11, respectively connected in parallel with resistors 50 and 51, are selected to insure a predetermined temperature characteristic of the output voltage Vd of the operational amplifier 204. As is apparent from the above description, the zero point adjustment and the zero point temperature characteristic compensation are carried out with the aid of the operational amplifier 204 and its peripheral resistors 8-13, 50, and 51.
With amplifying compensation, the adjustments are carried out in the order of: sensitivity temperature characteristic compensation; sensitivity adjustment; zero point temperature characteristic compensation; and zero point adjustment. The adjustments are separately performed. Hence, in the adjustments, high adjustment accuracy can be obtained relatively easily.
However, the number of elements in the circuit of FIG. 4 is large; four operational amplifiers, thirteen resistors, and three temperature dependent resistors are used. The large number of elements will obstruct miniaturization of the sensor, and will obstruct integration of all the elements of the sensor on a single silicon chip.
FIG. 5 shows another example of an amplifying compensation circuit having four compensation and adjustment functions. The output voltage of a bridge 100 consisting of strain gauges 101-104 is amplified by a differential amplifier made up of an operational amplifier 205 and resistors 15-18.
Temperature dependent voltage changes are compensated for by means of resistors 14 and 53 connected between the positive terminal of the power source and the strain gauge bridge. The resistor 53 has a negative temperature dependency. Resistor 53 may be a thermistor. The arrangement of resistors 14 and 53 gives a positive temperature dependency to the potential at the connecting point A of the strain gauges. The positive drive voltage developed between the supply voltage drive terminals of the bridge 100 compensates the negative temperature dependency of the pressure sensitivity of the strain gauges 101-104.
The resistance of the resistor 18 is selected to set the amplification to a predetermined value to adjust the sensitivity of the sensor.
Zero point temperature characteristic compensation is carried out by setting the resistance of the resistor 17 to compensate the temperature dependency of the zero point of the bridge 100 output.
Zero point adjustment is performed by adjusting the resistances of resistors 19 and 20 which are connected to the resistor 17 of the differential amplifier.
When the resistance of the resistor 17 is high, the positive temperature dependency of the bridge output in-phase potential is outputted unchanged, thus, giving a positive temperature dependency to the output V.sub.out of the circuit. As the resistance of the resistor 17 decreases, the potential at a connecting point B of the resistors 19 and 20, which do not have a temperature dependency, will affect the potential at the non-inversion input terminal of the differential amplifier 205, thus, making the temperature dependency of the sensor output V.sub.out more negative.
In the circuit of FIG. 5, the number of elements is not so large; one operational amplifier, seven resistors, and one temperature dependent resistor are used. However, the sensitivity adjustment, the zero point adjustment, and the zero point temperature characteristic are not performed separately. Without separation, it is difficult to perform the adjustments with high accuracy. Furthermore, the zero point temperature characteristic compensation range is narrow, because the range is limited by the temperature dependency of the bridge output in-phase voltage. Hence, compensation cannot be achieved if the zero point temperature dependency of the bridge output fluctuates greatly. In addition, because of the voltage drop across the parallel circuit of the resistors 14 and 53, the bridge drive voltage becomes lower than the sensor supply voltage decreasing the bridge output signal.
Japanese Patent Application (OPI) No. 217375/1984 (the term "OPI" as used herein means an "unexamined published application") discloses a circuit similar to the above-described circuit. The circuit disclosed has a smaller number of components than the circuit shown in FIG. 4. However, it is disadvantageous in that the zero point temperature compensation range is narrow, and the characteristic compensations are not separated from one another.